1. Field of the Invention
This invention relates in general to a method and apparatus for performing in file slider take-off measurements through tuned external acoustic emissions (AE) detection, and more particularly to a method and apparatus for determining the disk velocity when a slider no longer makes contact with a disk by mounting a vibration sensor to an actuator part without disassembly of the disk drive.
2. Description of Related Art
In the past few years, the computer industry has proven to be a vibrant part of the economy. People from all walks of life continue at a rapid pace to purchase personal computers and the software to run on them. Driving this explosion is the advent of the Internet and the ability to access this information through on-line services. Another reason for the increased use and demand for computers is the industry's move to make computer software more user friendly. As a result, some software packages can require 40 megabytes of free hard drive space just to load. However, the larger software size has created the need for hard drives that can hold more information.
The separation between the magnetic read/write element and the hard disk during operation, known as the flying height, is one of the important parameters that controls the performance and durability of a hard drive. In order to increase the recording density it is necessary to decrease the flying height so that the signal-to-noise ratio obtained from the read element is within an acceptable range. Ideally, zero spacing is preferred. However, zero spacing or contact recording leads to higher friction and wear at the head-disk interface, hence degrading the performance of the hard drive.
Today, hard drives typically have a fly height between 20 and 60 nanometers (nm). This fly height is on the same order of magnitude as the roughness of the disk. As hard drive manufactures incorporate proximity recording type sliders in their drives to achieve higher storage densities, considerable strain is created in controlling wear at the slider-disk interface. Further, ensuring proper take-off of the slider is important for reliable operation of disk files. The current manufacturing screen test for slider take-off is the high resolution Faraday (HRF) technique. However, the HRF technique allows only an indirect conclusion for the actual take-off velocity.
Acoustic emission (AE) has also become a popular method to monitor slider-disk interactions. AE sensors detect the energy associated with slider-disk contact via stress waves originating from the slider. These stress waves are associated with the slider resonant frequencies, and it is apparent that the acoustic emission signal contains detailed information about the type of slider-disk interactions, such as information about the impact behavior.
One such method is disclosed in Japanese Application 60-272065, filed Mar. 12, 1985, by Fumio Shiyoubuta, and which is assigned to Fujitsu Limited. In Shiyoubuta, the frequency of rotation of a magnetic disk due to a disk driving means is gradually increased. A slider head gradually separates from the surface of the magnetic disk due to the function of a slider. Using the contact with the fine projections of the magnetic disk prior to lift-off, a mechanical vibration is generated. The vibration is transferred through a gimbal to an acoustic emission (AE) sensor to produce an output signal. When the frequency of the rotation of a disk is increased, the slider-to-disk spacing increases and the decreasing mechanical vibration is transferred through the gimbal. As the vibration decreases, the distortion of the AE sensor approaches a small constant quantity. An AE output processing part receives the AE output signal to determine that an AE output decreases to the small constant value. The identification of this small constant value is associated with the magnetic head completely breaking free from the magnetic disk. However, Shiyoubuta provides only an indirect detection of slider take-off velocity based upon acoustic convergence to the small constant value. The actual take-off velocity is not determined. Further, the actual take-off velocity is not directly derived from the output of the AE sensor.
Another example of using acoustic emissions to monitor slider-disk interactions is disclosed in Japanese Application No. 57-022362, issued to Toshikatsu et al., and which is assigned to Fujitsu Limited. In Toshikatsu et al., an AE element is coupled to an actuator part outside the disk drive enclosure. The AE element is stuck there with an instantaneous adhesive. If a magnetic head is not floated sufficiently and rubs on the surface of the magnetic disc when the magnetic disc is rotated, a rubbing tone is generated and is transmitted through an arm and a rotation shaft until it reaches the AE element through a coil and stopper. The AE element outputs an electric signal corresponding to the rubbing tone. Consequently, the contacting condition between the magnetic disc and the magnetic head is discriminated on a basis of this output signal. Nevertheless, Toshikatsu et al. also provides only an indirect measurement of slider take-off velocity based upon the rubbing tone originating from friction between the slider and the disk surface. The take-off velocity is not directly derived from this rubbing tone. Furthermore, Toshikatsu et al. couples the AE element to an actuator part and consequently requires internal access to the file thereby preventing implementation after the disk drive has been fully assembled.
It can be seen that there is a need for a technique for directly measuring slider take-off velocity that does not increase the cost of disk drive production.
It can also be seen that there is a need for an AE sensing technique that may be implemented after assembly of a disk drive.
It can also be seen then that acoustic emission techniques are needed to determine the take-off velocity of a slider based upon a quantitative measurement of slider velocity without internal access to the file.